We are studying a unique core histone protein called macroH2A discovered in this laboratory. MacroH2A has a chimeric structure consisting of a region that closely resembles a full length histone H2A linked to a large nonhistone region. We examine the distribution of macroH2A in the nucleus of mammalian cells by confocal immunofluorescence microscopy using antibodies directed against the nonhistone region, and find two types of nuclear staining: large "blobs" and "speckles." By double labeling, we show that the distribution of macroH2A differs significantly fjrom that of DNA as well as nucleosomes in general, and that the blobs are not nucleolar. 8-31-96